Email delivery is slow between us and counterparty. What can I do to determine root cause?

Question

Emails between us and a specific counterparty are delayed. To give you some idea, we send 10 messages and over 40% arrive with a 20 (or greater) minute delay. We even had two messages arrive in 50 minutes!

I'm measuring the delay based on the header fields for the message. And I am only taking the delivery times between their and our perimeter SMTP servers.

I've also sent test messages via a 3rd party web mail account and can confirm that this isn't a slow internet link on our side, The test messages delivery times are sub-minute.

I don't think we're on RBLs as I would've expected messages to never reach us. I've been unable to do a traceroute from our perimeter SMTP server to theirs because of our firewall. If this issue does escalate we may have to put a device outside of the firewall to do traceroutes.

I also checked who their ISP was and I can confirm theirs is UK based (as is ours).

I'm at a loss to know how I can continue to determine the cause of these slow deliveries. I'd hate to go back to the counterparty's IT team and say it's a fault on your end, this seems like a cop out without hard evidence.

Just to recap:

• I am calculating the delay by measuring the time it takes for their perimeter SMTP server to deliver the message to our perimeter SMTP server. This means anti-spam/virus checks or mail journaling has already taken place on the counteparty's end and have yet to occur on our side.
• There are no delays if I send emails to our systems via a 3rd Party Web Mail account.
• Our userbase aren't complaining of slow Internet access or slow email delivery from other counterparties.

Thanks in advance!

Answer 1

Your SMTP logs should give you the information you need. They should tell you:

• If you are greylisted (some mailsystems require you to keep trying for a period of time to discourage spammers)
• If you are on an RBL (or at least if the receiving server recognizes it)
• If the receiving mailserver is too busy and is delaying reception
• How many attempts the sending server makes, and how often (perhaps you are only attempting to re-send every 20 mins?
• the exact time your mail was received by the receiving server

This should be all you need to determine the source of the delay.

Answer 2

if you can - take a look at logs of smtp outgoing server at the edge of your company and internet. does their MX server accept your messages immediately or do they get rejected at first? maybe your 3rd party counterpart is using greylisting ?

Answer 3

Have you tried telneting to their SMTP server and actually manually sending the appropriate SMTP commands.